This invention relates generally to turbine engine articles disposed about rotating articles, for example a turbine shroud, including a surface exposed to elevated temperature engine gas flow, and to their assemblies about rotating blades. More particularly, it relates to air cooled gas turbine engine shroud segments and to shroud assemblies, for example used in the turbine section of a gas turbine engine, especially segments made of a low ductility material.
Typically, a plurality of gas turbine engine stationary shroud segments assembled circumferentially about an axial flow engine axis and radially outwardly about rotating blading members, for example about turbine blades, defines a part of the radial outer flowpath boundary over the blades. As has been described in various forms in the gas turbine engine art, it is desirable to maintain the operating clearance between the tips of the rotating blades and the cooperating, juxtaposed surface of stationary shroud segments as close as possible to enhance engine operating efficiency. Some examples of U.S. patents relating to turbine engine shrouds and such shroud clearance include U.S. Pat. No. 3,798,899—Hill; U.S. Pat. No. 3,807,891—McDow et al.; U.S. Pat. No. 5,071,313—Nichols; U.S. Pat. No. 5,074,748—Hagle; U.S. Pat. No. 5,127,793—Walker et al.; and U.S. Pat. No. 5,562,408—Proctor et al.
Metallic type materials currently and typically used as shrouds and shroud segments have mechanical properties including strength and ductility sufficiently high to enable the shrouds to be restrained against typical movement, deflection and/or distortion resulting from thermal gradients and other pressure forces known to occur in operation of a turbine engine without detrimental effect on the shroud material. Examples of such restraint include the well known side rail type of structure, or the C-clip type of sealing structure, for example described in the above identified Walker et al patent. That kind of restraint and sealing results in application of a compressive force at least to one end of the shroud to inhibit chording or other distortion. Other patents, such as the above-identified McDow et al. patent, describe radial clearance control between juxtaposed engine members such as rotating blades and surrounding stationary structure aerodynamically loaded against each other during engine operation. Such active radial clearance control is responsive to changes in temperature during engine operation. In some of such patents, for example the McDow et al. patent, description is included for a partial axial movement of an entire assembly of adjacent, juxtaposed, contacting engine members, for example the assembly of adjacent stationary turbine vanes and juxtaposed or intermediate shrouds held in contact therewith. Such axial movement, that occurs as a result of adjacent members applying pressure on an adjacent member such as a shroud during engine operation, can result in application of significant pressure to a shroud or shroud segment.
Current gas turbine engine development has suggested, for use in higher temperature applications such as shroud segments and other components, certain materials having a higher temperature capability than the metallic type materials currently in use. However such materials, forms of which are referred to commercially as a ceramic matrix composite (CMC), have mechanical properties that must be considered during design and application of an article such as a shroud segment. For example, as discussed below, CMC type materials have relatively low tensile ductility or low strain to failure when compared with metallic materials. Also, CMC type materials have a coefficient of thermal expansion (CTE) in the range of about 1.5-5 microinch/inch/° F., significantly different from commercial metal alloys used as restraining supports or hangers for shrouds of CMC type materials. Such metal alloys typically have a CTE in the range of about 7-10 microinch/inch/° F. Therefore, if a CMC type of shroud segment is restrained or axially loaded during engine operation, and cooled on one surface as is typical during operation, compressive forces can be developed in a CMC type segment sufficient to cause failure of the segment.
Generally, commercially available CMC materials include a ceramic type fiber for example SiC, forms of which are coated with a compliant material such as BN. The fibers are carried in a ceramic type matrix, one form of which is SiC. Typically, CMC type materials have a room temperature tensile ductility of no greater than about 1%, herein used to define and mean a low ductility material. Generally CMC type materials have a room temperature tensile ductility in the range of about 0.4-0.7%. This is compared with metallic shroud and/or supporting structure or hanger materials having a room temperature tensile ductility of at least about 5%, for example in the range of about 5-15%. Shroud segments made from CMC type materials, although having certain higher temperature capabilities than those of a metallic type material, cannot tolerate the above described and currently used type of compressive force or similar restraint force against chording. Neither can they withstand a stress rising type of feature, for example one provided at a relatively small bent or filleted surface area, without sustaining damage or fracture typically experienced by ceramic type materials. Furthermore, manufacture of articles from CMC materials limits the bending of the SiC fibers about such a relatively tight fillet to avoid fracture of the relatively brittle ceramic type fibers in the ceramic matrix. Provision of a shroud segment assembly, in one embodiment including shroud segments of such a low ductility material, floating axially independently of other engine members and positioned or disposed in a manner that does not apply detrimental force to the shroud segment during operation would enable advantageous use of the higher temperature capability of CMC material for that purpose.